Evaluation of urban PM2.5 concentrations over 73 major cities and their association with satellite Aerosol Optical Depth: A global analysis of ambient air pollution

Air pollution is a notable environmental health risk factor in modern society, affecting 99 % of the population. In the current study, we investigate the global fine Particulate Matter (PM2.5) urban footprint based on PM2.5 surface measurements from 76 AirNow stations worldwide from January 2019 until March 2024 and investigate the association with satellite-based Aerosol Optical Depth (AOD). All 76 stations in all years, with no exception, exceed the World Health Organization (WHO) annual average air quality guideline level (i.e., 5 μg/m 3 ), and 29 stations exceed the WHO annual average interim target 1 (i.e., 35 μg/m 3 ). The global map of the mean PM2.5 concentrations indicate a hotspot in South Asia where these concentrations sustain considerably high levels. Disaggregation by World Bank country income level groups reveals income inequality with a gap between the lower-middle (median 42.1 μg/m 3 ) and the upper-middle (22.5 μg/m 3 ) income countries. Site-specific correlations on PM2.5-AOD pairs from AirNow and the Moderate Resolution Imaging Spectroradiometer (MODIS), respectively, for statistically significant observations show a large variation between stations, with R ranging between −0.60 and 0.79. In a case study over Vientiane, Laos, the Geostationary Environment Monitoring Spectrometer (GEMS) and MODIS AOD satellite products indicate a considerably higher number of GEMS observations available due to intra-day acquisition capability. A closer look at the GEMS AOD-PM2.5 hourly pairs show a clear diurnal pattern. The results of this large-scale experiment showcase the robustness and applicability of the synergistic use of the global AirNow surface monitoring system and satellite products. • All 73 cities exceed the WHO annual average PM2.5 air quality guideline level. • PM2.5 is higher in lower-middle compared to upper-middle income countries. • Correlation between surface PM2.5 and satellite AOD varies greatly between locations. • GEMS AOD over Vientiane is more accurate between 09:00–11:00 local time.